Apparatus for converting sound to frequency modulated currents



W. HAUSZ Oct. 2, 1945.

APPARATUS FOR CONVERTING SOUND TO FREQUENCY MODULATED CURRENTS FiledMarch 9, 1943 Inventor: Walter Hausz, y 36 6 His Attovne g Patented Oct.2, 1945 APPARATUS FOR CONVERTING SOUND TO FREQUENCY MODULATED CURRENTSWalter Hausz, Schenectady, N. Y., asslgnor to General Electric Com NewYork pany, a corporation of Application March 9, 1943, Serial No.478,565

11 Claims.

My invention relates to sound reproducing apparatus, and it has for oneof its objects to provide an improved honograph pick-up apparatus forconverting recorded sound waves to frequency modulated electric waves.

My invention relates more particularly to phonograph pick-up apparatusof the type in which one electrode of a variable capacitor is caused tovary in response to the amplitude of motion of a stylus which cooperateswith or engages the sound record to be reproduced. It is desirable thatsuch apparatus be of light weight in order to reduce wear on recordingsbeing used and to provide a constant response in the output of theapparatus for a constant amplitude of needle motion over the frequencyrange of the recorded sound waves.

A further object of my invention is to provide an improved pick-upapparatus of the above type in which the driving force of the record onthe pick-up apparatus is maintained at a minimum value over the entireuseful frequency range of the recorded sound waves.

Another object of my invention is to provide an improved phonographpick-up apparatus for converting recorded sound waves to frequencymodulated electric waves in which response to constant and low frequencyforces on the apparatus and due to eccentricities in the recording issubstantially eliminated.

A still further object is to provide an improved sound pick-up apparatusin which excessive resonance in the useful frequency range issubstantially avoided.

One of the features of my invention is the use of a sound wave pick-upapparatus as means for frequency modulating a pair of electron dischargedevices, connected in push-pull and oscillating at differentfrequencies, and utilizing the detected difference frequency of theiroutput as a source of frequency modulated carrier waves. Another objectof my invention, therefore, is to provide an improved frequencymodulation system in which distortion of the output wave caused byharmonics of the modulating signal is reduced to a very low level.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,both as to its organization and method of operation, together withfurther objects and advantages thereof, may best be understood byreference to the following description taken in connection with theaccompanying drawing, in which Fi 1 is a pick-up apparatus constructed(Cl. I'm-100.4)

according to my invention; Fig. 2 is a sectional view of the apparatusof Fig. 1 along the line 2-2; Fig. 3 shows an element of the pick-updevice of Fig. l; and Fig. 4 is a circuit diagram of a portion of afrequency modulation system utilizing the pick-up device of Fig. 1.

Referring to the drawing, I have shown in Fig. 1 a light weightphonograph pick-up of the variable capacity type and comprising avariable plate or electrode I0, a pair of fixed or stator electrodes Hand I2, and a needle 13. The electrode I0 comprises a sheet of elasticmaterial having a pair of conductive surfaces and formed with a pair ofend members or legs l5 and 16, a central member or leg I l, and a crossmember I8 connecting the legs l5 and 16 with the central member ll. Theplate l0 may be formed from a sheet of elastic metal, such as berylliumcopper for example, or may comprise a thin layer of plastic material ofthe synthetic resin type having surfaces made conductive by spraying orplating with a conductivematerial, such as graphite. The legs l5 and I6may be formed by cutting a pair of slots, one of which is indicated atIS in Fig. 1 and both of which are indicated at l9 and 20 in Fig. 3, inthe sheet of material and drilling a pair of holes 2| and 22, asindicated in Fig. 3, at the base of these slots to prevent localconcentration of stresses in the material as the central member l! isflexed in a manner pointed out later. The leg members l5 and I6 likewiseextend vertically from the cross member I8 a greater distance than doesthe central member H. In this manner the legs l5 and I6 provide meansfor rigidly attaching the variable plate to the usual phonograph pick-uparm 24. The plate It), likewise, is formed with an edge 25 bent at rightangles to the cross member I8 to give rigidity to the plate, as well asto provide means for attaching the pick-up needle l3 to the plate 30.Preferably, the needle i3 is of very light construction and maycomprise, for example, a sapphire needle integrally attached to the edgeor stiffening member 25.

The stator plates H and I2 are formed of a substantially rigid materialhaving conductive surfaces, such as a metal plate or a plastic plate,thicker than the member l0 and provided with a conducting surface in thesame manner as the plate Ill. The plates H and l2 are of substantiallythe same size as the central member I! and are supported in spacedparallel relation with member I! by means of a pair of insulatingmembers 30 and 3|. The members 3}] and a minimum of oscillations, andmay comprise a synthetic resin, such as, for example, butyl rubher. Theinsulators 30 and 3| may be attached to the plates ID, i! and I2 by anysuitable method, such as by vulcanization. Leads 32 and 33 are attachedto plates II and I2 to provide means for connecting these plates to theelectrical circults of thepick-up.

In the construction thus far outlined, the plates Ill and II and theplates in and I2 form a plurality of capacitors, the capacitances of theindividual capacitors being varied in opposite sense by movement of thevariable plate I as the needle I3 moves along the usual sound track 35of a phonograph record. Movement of the needle I! along the track 35causes the central portion I! of plate ill to vibrate at audiofrequencies. Preferably, the mass of the plates H and I2 and theinsulators 30 and 3! is such that the natural frequency of vibration ofthese plates in conjunction with the plate occurs at a frequency belowthe range of frequencies of the usual audio impulses on a phonographrecord. Thus, at frequencies in the range of the audio .impulses, thetop of the mass comprising the stators H and I2, the insulators 3d and3| and the upper portion of member l'l, remains substantially fixed inspace and only the lower portion of plate It moves in response to theaudio frequency impulses imparted to needle l3 by sound track 35.Impulses of sub-audio frequencies at or near the natural frequency donot cause excessive motion of plates H and I2 by virtue of the dampingproperties of insulators 30 and 3|.

While the plates II and I2 do not move in response to frequencies in theaudio range, application of a constant force or a force of low frequencyto needle I3, because of the presence of insulators 30 and 3 l iseffective to move the plates III, II and I! as a unit. In this mannerapplication of a sub-audio frequency force, which may be caused byoff-center records or other eccentricities in a recording, does notchange the capacitances of the component capacitors of the pick-upapparatus. At frequencies considerably above the natural frequency ofplates II), II and l2, a constant amplitude of needle motion produces aconstant percentage change in capacity. At high frequencies in the audiorange, moreover, the response is of substantially the same order as forcorresponding forces at lower frequencies in the audio range because ofthe construction of the pick-up device. Thus, it is apparent that theonly mass which is moved by the vibrating needle I3 is the mass of theneedle plus a small portion of the mass of the vibrating plate or reedl0. In this way, over the entire useful frequency range, the drivingforce required to obtain a desired capacitance change is kept to avalue. The construction outlined, moreover, avoids all violentresonances of the vibrating plate even for frequencies outside of theuseful frequency range, since the insulators 30 and 3| are chosen tohave suflicient damping because of their high mechanical hysteresisproperties.

In the circuit of Fig. 4, there is shown a portion of a frequencymodulated high frequency apcircuits caused by harmonics of the audioimpulses. In the circuit, electron discharge devices 40 and 4| are twooscillators providing high frequency oscillations determined by thetuned oscillatory circuits connected between the anodes and controlelectrodes of these devices. For reasons which are pointed out later,these oscillatory circuits, which comprise the inductances 42 and 43 andoapacitances 65 and 66, are tuned to different frequencies. One terminalof the inductances 42 and 43 is grounded and connected through capacitor44 and load resistances 45 and 46, respectively, to the anodes ofdevices 40 and M. The opposite terminals of these inductances areconnected through grid resistors 41 and grid by-pass capacitors 48 tothe control electrodes of these devices and intermediate points 49 oninductances 42 and 43 are connected to the cathodes of the devices.Operating potential for the anodes of devices 40 and 4! may be suppliedfrom any suitable source, such as the battery 5|).

The capacitances 65 and 66 are formed by the plates IO, N and I! of thepick-up devices. In the absence of audio impulses to vibrate the plateill, the devices 40 and 4| individually function as the usual type ofsingle tube oscillator in which a portion of the energy in the anodecircuits is fed back to the input circuits to maintain oscillations inthe output circuits of the tubes. Since the capacitors 65 and 66 formpart of the tuned circuits of oscillators 4|) and 4|, when the reed orcommon plate I0 is moved to one side, for example to the right-handside, by a sound impulse imparted to needle l3, the frequency ofoscillator 40 increases and that of oscillator 4| decreases. In thismanner the oscillations in the output cirsuits of oscillators 4| and 4Bare frequency modulated by audio impulses supplied to the needle l3.

In order to obtain a. minimum of harmonic distortion in the output ofthe frequency modulated oscillators 40 and 4 I, the anodes of devices 40and 4| are coupled by means of capacitors 52 and 53 to a resistor 54,the mid-point of which is connected to ground through a resistor 55. Adiode 55 is likewise electrically connected between the mid-point ofresistor 54 and ground through a load resistor 51. This diode 56functions in a well known manner as a detecting circuit to developacross resistance 51 voltages of the frequency of oscillations ofoscillators 40 and 4|, as well as voltages having frequencies equal tothe sum and difference of frequencies of the oscillations of oscillators40 and 4|. In order to remove from the output terminals 60 and 6| of thefrequency modulation system unidirectional voltage components, voltagesof the frequencies of oscillators 40 and 4|, and voltages havingfrequencies equal to the sum of the frequencies of these oscillations, afilter circuit comprising inductance 62 and capacitances 63 and 64 isconnected across resistance 51 between the cathode of diode 55 andground. The resulting voltages supplied to the output terminals 60 andBI contain the difference frequency between the two oscillators. Thisfrequency is lower than the frequency of either of the oscillators andthese oscillations are frequency modulated in accordance with thedifference in capacity which occurs between the two pick-up capacitors65 and 66 as the reed I0 is vibrated at an audio rate. One of theadvantages of this particular system is that, since a larger amount ofmodulation is 'ob'-" tained for a given capacity change, the use 'ofvibrations harmonics of the audio signal present on sound track 35 inthe output of oscillators 4| and 40 by the push-pull connection ofcapacitors 65 and 66 in the tuned oscillatory circuits.

It is, of course, apparent that since the two oscillators 40 and 4|differ in frequency, the desired output frequency for perfectcompensation and cancellation of even harmonics in the output of theoscillators is achieved only by properly constructing the capacitors 65and 66 or by choosing the additional tuning capacitors 61 and 68 so thatthey have capacitances proportioned to the desired frequencies. Ingeneral, it may be necessary to use the additional tuning capacitors 61and 6B in order to obtain oscillations of desired frequencies. Inspecific cases, one or both of these capacitances may be reduced to zerovalue.

While I have shown a pick-up device in which two stators H and B2 areused in conjunction with a vibrating element ill, it is apparent thatthe principles pointed out in connection with the construction andoperation of the pick-up of Fig. 1 may be applied equally as well to acase in which a single stator element is used in conjunction with thevibrating reed. In a frequency modulation system using a pick-up devicehaving a single stator plate, of course, a single oscillator is usedrather than a plurality of oscillators Gil and ll shown in Fig. 4. Insuch a system, harmonic distortion in the output may be reduced byincreasing the separation between the stator element and the reedelement to reduce the per cent capacity change for vibrations of a givenamplitude.

While I have shown particular embodiments of my invention, it will, ofcourse, be understood that I do not wish to be limited thereto sincevarious modifications may be made, and l contemplate by the appendedclaims to cover any such modifications as fall within the true spiritandscope of my invention.

What i" claim as new and desire to secure by Letters Patent of theUnited States is:

i. In combination, a member vibrating at audio frequencies, a pair ofoscillatory circuits tuned to different frequencies, means for frequencymodulating oscillations of said circuits by the vibrations of saidmember comprising a pair of capacitors having a common electrode, eachof said capacitors being connected in a respective one 01 said circuits,means to vibrate said common electrode inaccordance with the vibrationsof said member, and means coupled to said circuits for derivingtherefrom voltages of a frequency equal to the difference in frequencyof said oscillations.

2. Apparatus for converting audio frequency into frequency modulatedwaves comprising a pair of electron discharge oscillators having inputand output circuits, said output circuits being connected in push-pull,a respective one of a pair of tuned oscillary circuits coupled to theinput circuit of each of said oscillators, each of said oscillatorycircuits being tuned to a different frequency and including acapacitance, means responsive to said vibrations for varying saidcapacitances in opposite sense for frequency modulating saidoscillators, and means coupled to said output circuits for detecting thedifference frequency of said modulated oscillations.

3. The combination, in a capacity pick-up device for phonographs, of apair of electrodes, one of said electrodes being supported from theother of said electrodes by insulating means, a stylus attached to theother of said electrodes adapted to engage a record to be reproduced andto vibrate said other electrode in accord with said record to bereproduced, both of said electrodes and said insulating means beingmoved in unison by motions of said stylus at frequencies below thefrequencies involved in said record to be reproduced, and saidelectrodes having a mass and said insulating means having mechanicalhysteretic properties such that said other electrode vibrates relativeto said one electrode in response to motions of said stylus atfrequencies involved in said record to be reproduced.

4. In combination, a variable capacitor comprising a thin flexible platehaving a pair of conductive surfaces, said plate comprising a pair ofend members and a central member connected by a cross member, a pair ofsubstantially rigid plates in parallel spaced relation with said centralmember, means for supporting said rigid plates from said central member,said means including insulating members located between a correspondingedge of said central member and said plates, means attached to saidcross member for moving said central member with respect to said rigidplates at audio frequencies, and means whereby said capacitor may besupported from said end members.

5. A pick-up device for the reproduction of recorded impulses comprisinga fiat plate of thin flexible material having a conductive surface, saidplate comprising a pair of legs connected together at one end, asubstantially rigid plate having a conductive surface in spaced parallelrelation with one of said legs and forming therewith a capacitanceelement, an insulating member joining said rigid plate and the end ofsaid one leg opposite said one end, said rigid plate being supportedsolely by said one leg through said insulating member, means attached tosaid one end for actuating said one leg to vary the capacitance of saidelement in response to said impulses, and supporting means attached tothe other of said legs at the end opposite said one end.

6. The combination, in a phonograph pick-up apparatus, of a pair ofcapacitors having outer electrodes and a common electrode positionedbetween said outer electrodes, said electrodes being fixedly attachedtogether along one edge thereof and insulated from each other, and allof said electrodes being movable together about said one edge inresponse to movement of the opposite edge of said common electrode atfrequencies lower than the frequencies to be reproduced, a stylusattached to said opposite edge adapted to engage the record to bereproduced and to vibrate said opposite edge in accord with said recordthereby to vary differentially the capacitances of said capacitors atthe frequencies to be reproduced, and means to produce electric wavesvarying with the difference in said capacitances whereby said waves varyin accord with the record to be reproduced and are unaffected by lowfrequency forces affecting said stylus.

7. The combination, in a capacity pick-up device for phonographs, ofthree parallel electrodes having capacity therebetween, a stylusattached to the, intermediate one of said three electrodes to vibratesaid electrode between th other two of said three electrodes in accordwith the record to be reproduced, and a mechanical coupling between saidelectrodes to vibrate both of said two electrodes in unison with saidinteraccord with the record to be reproduced thereby to vary oppositelythe capacities between said intermediate plate and the respective otherof said plates, and a mechanical coupling between said plates to vibrateall of said plates in unison and in phase in response to vibration ofsaid intermediate plate at frequencies lower than frequencies to bereproduced thereby to reduce capacity variations in said capacities atsaid lower frequencies.

9. The combination in a pick-up device, of a pair of plates, one of saidplates being supported at one edge thereof and carrying a stylus at theother edge, means insulatingly supporting said other plate from saidfirst plate to vibrate in unison therewith at a certain frequency, saidmeans including means whereby said stylus vibrates said first plateindependent of said other plate at higher frequencies.

10. The combination, in a pick-up device, of a pair of conductiveplates, one of said plates being supported at one edge thereof andcarrying a stylus at its other edge to engage a record to be reproduced,means insuiatingly supporting said other plate from said one edge inspaced parallel relation with said first plate, whereby said platesvibrate in unison, at frequencies below a certain frequency, said meansincluding means whereby said stylus vibrates said first plateindependent of said other plate at frequencies greater than saidfirstfrequency.

11. The combination, in a capacity pick-up device for phonographs, of apair of electrodes, one of said electrodes carrying a stylus adapted toengage a record to be reproduced and mounted for vibration relative tosaid other electrode in accord with said record to be reproduced,thereby to vary the capacity between said electrodes in accord with saidrecord, and means to prevent relative movement between said electrodesin response to slow motion of said stylus at frequencies below thefrequencies involved in the record to be reproduced, said meanscomprising'means to support said other electrode from a portion of saidone electrode remote from said stylus whereby the portions of bothelectrodes near said stylus vibrate freely.

WALTER HAUSZ.

